662 



PHYSIOLOGY. 



ends of the nerve. Bert's experiment of fixing the end of a rat's 

 tail in a wound in the back and dividing the tail at its root after 

 union has ensued shows that the stimulus is transmitted both ways 

 in the case of sensory nerves. When the root of the divided tail was 

 irritated there followed symptoms of pain, showing that the nerve 

 impulse of sensation was transmitted in a direction opposite to the 

 normal one. 



This fact is somewhat difficult of explanation, but in support of 

 it comes Kiihne's classical experiment. This investigator takes the 

 sartorius muscle of a frog and separates it lengthwise, beginning at 



a I 



Fig. 265. Curves Illustrating the Measurement of the Velocity of a 

 Nervous Impulse (Diagrammatic). (FOSTER.) 



To be read from left to right. 



The same muscle-nerve preparation is stimulated (1) as far as possible from 

 the muscle and (2) as near as possible to the muscle; both contractions are 

 registered by the pendulum myograph exactly in the same way. 



In 1 the stimulus enters the nerve at the time indicated by the line a, the 

 contraction, shown by the dotted line, begins at b' ; the whole latent period 

 therefore is indicated by, the distance from a to I)'. 



In 2 the stimulus enters the nerve at exactly the same time (a) ; the con- 

 traction, shown by the unbroken line, begins at 6; the latent period therefor is 

 indicated by the distance between a and 6. 



The time taken up by the nervous impulse in passing along the length of 

 nerve between 1 and 2 is therefore indicated by the distance between & and b', 

 which may be measured by the tuning-fork curve below. 



N. B. No value is given in the figure for the vibrations of the tuning-fork, 

 since the figure is diagrammatic the distance between the two curves, as com- 

 pared with the length of either, having been purposely exaggerated for the sake 

 of simplicity. 



its extremity, so that two small tongues are formed. Each tongue 

 receives nervous filaments from the same peripheral branch. If one 

 of these small tongues be mechanically stimulated the exciting state 

 of the motor nervous fiber is found to be communicated to the other 

 small tongue. Since the second small tongue was excited by a motor 

 stimulus to the first one, it follows that the conduction occurred in a 

 centripetal direction along the course of a motor nerve. This direc- 

 tion is different from that of normal conduction, for the nerve which 

 has been thus excited is a centrifugal motor nerve. Therefore, since 

 the motor nerve has played the role of a centripetal conductor in this 



